(1) Field of the Invention
The present invention relates to a water-in-oil emulsion explosive composition containing microvoids, and more particularly relates to a water-in-oil emulsion explosive composition containing specifically limited hollow microspheres as microvoids, having a high consistency without deteriorating the storage stability in initiation sensitivity in a small diameter cartridge (25 mm diameter), and being able to be easily handled in the charging at the blasting.
(2) Description of the Prior Art
There have hitherto been used various microvoids in the water-in-oil emulsion explosive composition in order to decrease the specific gravity of the explosive composition and to improve the detonation properties, such as initiation sensitivity, propagation property of detonation and the like.
The term "microvoids" herein used means hollow microspheres, bubbles formed by a foaming agent and bubbles mechanically (physically) blown into the explosive composition.
When microvoids contained in a water-in-oil emulsion explosive are bubbles, the consistency of the explosive is low, and further the bubbles disappear due to the lapse of long time, resulting in the increase of the specific gravity of the explosive and in the noticeable decrease of the detonation sensitivity of the explosive.
When microvoids contained in a water-in-oil emulsion explosive are hollow microspheres, that is, when the microvoids are inorganic hollow microspheres produced from alkaline or weakly alkaline glass, such as sodium borosilicate, sodium calcium borosilicate or the like, the alkaline or weakly alkaline glass dissolves out in water during the kneading due to its high solubility in water, and hence the resulting water-in-oil emulsion looses the balance in water-in-oil emulsion, increases its consistency, is difficult in handling and is poor in storage stability. Moreover, the cost of raw material per unit volume of the resulting emulsion explosive composition is high.
The use, as hollow microspheres, of neutral or weakly acidic hollow microspheres, such as inorganic hollow microspheres produced from, for example, shirasu (shirasu is one kind of volcanic ash); carbonaceous hollow microspheres produced from, for example, pitch; synthetic resin hollow microspheres produced from, for example, vinylidene chloride-acrylonitrile-methyl methacrylate terpolymer (hereinafter, refered to as Saran (registered trademark of the terpolymer sold by Dow Chemical Co.) or phenolic resin; and the like, results in a water-in-oil emulsion explosive composition having a low consistency, and the explosive is difficult in handling and has a poor storage stability in initiation sensitivity in a small diameter cartridge.
Further there have been proposed methods for improving the storage stability in initiation sensitivity in a small diameter cartridge (25 mm diameter) and at low temperatures of water-in-oil emulsion explosives by adding to their disperse phase a compound of metals having an atomic number of at least 13 and being other than the metals of Groups I and II of the Periodic Table, a water-soluble strontium compound or an organic builder and/or inorganic builder (inorganic acid salt, organic acid salt and chloride of ammonium, alkali metal and alkaline earth metal, a part of which may be replaced by hydrogen) in U.S. Pat. No. 3,715,247 and No. 3,765,964 and Japanese Patent Laid-open Specification No. 42,952/82 and No. 47,791/82. However, in these explosives, ones containing conventional hollow microspheres are low in consistency and are difficult in handling as well.
As for the combustible material which forms a continuous phase in a water-in-oil emulsion explosive composition, when a large amount of oil or emulsifier having a high melting point or high softening point is contained in an explosive composition or the content of the oil or emulsifier therein is adjusted, a water-in-oil emulsion explosive composition having a high consistency can be obtained. However, the use of a large amount of oil or emulsifier having a high viscosity lowers the storage stability in initiation sensitivity of the resulting water-in-oil emulsion explosive composition. While, a water-in-oil emulsion explosive composition having a low consistency, particularly a small diameter cartridge formed of the explosive composition, deforms during the transportation or deforms at the charge into a borehole to cause difficulties in the charging. That is, the explosive having a low consistency is difficult in handling, is poor in blasting effect and is often misfired and remains. Further, in a water-in-oil emulsion explosive composition containing a large amount of oil or emulsifier, particles which form the disperse phase are apt to be connected to each other or crystallized due to the lapse of time and other external factor, resulting in the breakage of the water-in-oil emulsion, and hence the explosive composition is poor in storage stability in initiation sensitivity, particularly, in a small diameter cartridge (25 mm diameter).
The inventors have made various investigations for a long period of time in order to overcome the drawbacks of water-in-oil emulsion explosive compositions containing the above described conventional hollow microspheres, and found out that the use of specifically limited hollow microspheres can produce a water-in-oil emulsion explosive composition having a high consistency without deteriorating the storage stability in initiation sensitivity in a small diameter cartridge (25 mm diameter), and hence the explosive composition can be easily handled. As the result, the present invention has been accomplished.